Study for online range monitoring with the interaction vertex imaging method

Abstract

Ion beam therapy enables a highly accurate dose conformation delivery to the tumor due to the finite range of charged ions in matter (i.e. Bragg peak (BP)). Consequently, the dose profile is very sensitive to patients anatomical changes as well as minor mispositioning, and so it requires improved dose control techniques. Proton interaction vertex imaging (IVI) could offer an online range control in carbon ion therapy. In this paper, a statistical method was used to study the sensitivity of the IVI technique on experimental data obtained from the Heidelberg Ion-Beam Therapy Center. The vertices of secondary protons were reconstructed with pixelized silicon detectors. The statistical study used the [Formula: see text] test of the reconstructed vertex distributions for a given displacement of the BP position as a function of the impinging carbon ions. Different phantom configurations were used with or without bone equivalent tissue and air inserts. The inflection points in the fall-off region of the longitudinal vertex distribution were computed using different methods, while the relation with the BP position was established. In the present setup, the resolution of the BP position was about 4-5 mm in the homogeneous phantom under clinical conditions (10 incident carbon ions). Our results show that the IVI method could therefore monitor the BP position with a promising resolution in clinical conditions.

Investigation of Suitable Detection Angles for Carbon-Ion Radiotherapy Monitoring in Depth by Means of Secondary-Ion Tracking

“…A dataset equivalent to eight mini-trackers with an active area of 2 cm 2 each was used to detect and localize an air cavity of 2 mm thickness and 80 × 80 mm 2 transverse area from different detection angles relative to the beam axis. The size of the investigated inter-fractional change is considerably smaller than in previously published studies that used inserts with a thickness of 10 mm ( 11 ), 28.5 mm ( 29 ) and 28 mm ( 30 ).…”

Intra- and inter-fraction relative range verification in heavy-ion therapy using filtered interaction vertex imaging

“…This change alone reduces the number of required ions from the 1.29  10 10 delivered by the highest-intensity 50 pA beam and longest delivery time of 330 s to only 1.52  10 7 . The better-than-expected improvement is attributed to the larger detectors measuring secondary particles exiting the patient at smaller angles relative to the primary beam axis, which are significantly more likely to be produced (Finck et al 2017). This simulation does assume full efficiency for the segmented detectors, while an efficiency correction is applied to the PSD output; however, efficiency differences may be resolved by adding an additional tracker arm, or making small adjustments in tracker angle to take advantage of the increased secondary particle yield at smaller off-axis angles.…”

“…Trackers were positioned at a 45 degree angle from the primary beam axis, as shown in Figure 2 a). This angle was selected based on the known forward bias of secondary particles (Finck et al 2017), to provide a compromise between maximizing total counts and maintaining a sufficiently low count rate to reduce dead time. Larger off-axis angles also provided less longitudinal uncertainty, for geometric reasons.…”

“…Another key challenge in IVI is in collecting data at clinical dose rates. Segmented detectors again provide an advantage in allowing higher count rates due to faster charge collection, although the success of fIVI with lower-resolution PSDs indicates that segments can be much larger than the 18.4 to 55 μm square pixels used in previous studies (Gwosch et al 2013, Finck et al 2017, Reinhart et al 2017, Félix-Bautista et al 2019. Using a strip-segmented detector of modest pitch, such as 300 μm, is sufficient to break the surface of even a large 10 cm  10 cm detector into segments smaller than the full position-sensitive detector used in this study.…”

“…The fIVI method, like all implementations of IVI, is a noninvasive monitoring technique, requiring no implanted markers or injected tracers. Because IVI collects all necessary data during treatment, no additional time is required of the patient for post-treatment verification scans (Henriquet et al 2012, Gwosch et al 2013, Finck et al 2017. These benefits would allow fIVI to be used for online range monitoring of every fraction in a treatment plan.…”